As for the saying of the Christians: “He [Īsá] was a god by his divine nature and a messenger by his human nature”, this is an absolutely false claim. This is clarified in the following points:

1. The one who used to speak to mankind was either Allāh Himself or His messenger. If it was Allāh, then it cannot also be the messenger of Allāh. If it was the messenger of Allāh, then it cannot also be Allāh.For this reason, when the One who spoke to Mūsá (عليه السلام) from the direction of the tree was Allāh himself, there are no revelations that refer to him being spoken to by a messenger of Allāh. This is an irrefutable point that cannot be answered regardless of the manner they choose to interpret this unity [between god and man].

   For, indeed, it is a well-known fact that the people used to listen to the Messiah (عليه السلام) speak with his own voice which they fully recognised. Neither his voice, nor his state differed during his speech in the way that a person’s speech would change after being possessed by a Jinn which leaves him if the Jinn was to speak using the tongue of the possessed person. There is a plainly evident difference between the speech of the possessed and the unpossessed. The state of the possessed also differs, just as the state of his speech differs from one’s normal state. There is certainty in the fact that the words he speaks in his possessed state are unknown to him, words of a person whose mind has abandoned him. This is most apparent to anyone in his presence, as his voice and intonation changes. How, then, could we not say the same for the one who has allegedly united with the Lord of the worlds, who speaks with His words? In such a state, there must undoubtedly be a difference between his words and voice, and the words of the rest of humankind and their voices. The extent of this difference would be even greater than the difference between the possessed and the unpossessed. Rather, this difference should be unquantifiable.

   This may be further clarified by the fact that when Mūsá (عليه السلام) heard the voice of Allāh, it was a miracle that represented a disruption to the norm, different to any other voice he heard prior. Then, he saw signs in the form of miracles and surprising occurrences which further clarified to him that none could have spoken with the voice he heard except Allāh. As for the Messiah, there was no difference between his speech or voice for the entire duration of his life and the speech of anyone else that would indicate that he was even a prophet, much less that would prove he was a god. It was only known that he was a prophet by means of separate, independant evidences. His state never changed despite their claim that this unification (between him and god) always existed from the time his human nature was formed in the womb of his mother Maryam, and never ceased. That is, his (allegedly) divine nature never abandoned his human one. It is well-known that when he addressed the people, if the statements made were that of the Lord of the Worlds then it is an impossibility for him to also be a messenger (of God simultaneously). If the statements delivered were from a messenger, then the voice with which he spoke was not that of the Lord of the Worlds.

2. His delivery to the people (i.e. address) was that of a prophet and messenger. This was authentically narrated from him in several places [in the Bible and the Qurʾān].
3. The unification of two different entities into one singular entity while both entities remain in their original state—without transformation or mixing of any kind—is completely, intellectually, impossible. The only intellectually sound form of unification is that which occurs in the presence of transformation and mixing, like water with intoxicants or milk. If the two entities become a single one, they both transform and mix with one another.
4. With unification, two entities become one such that God is both the messenger and the messenger is God simultaneously. That is, they are one and the same. However, if God is not the messenger, they must be considered two separate entities. This is a fact regardless of the parables they use to communicate this, like iron in fire or the soul in the body. This only proves the corruption of their intellects. For when iron is removed and hammered or plunged into water, its associated fire is affected. Just as if the body was to feel hunger, or was crucified and felt pain, this pain similarly affects the soul. Therefore, based on their claim, the Lord of the worlds must feel the pain of hunger, thirst, being beaten and crucified. This is an even viler claim than that of the Jews who claim that He is a poor miser, or that He was affected by fatigue.

Source: Al-Jawāb al-Ṣaḥīh 2:46-48
Translated by: Riyāḍ al-Kanadī